Date: 201512
Local time of day: 1801-2400
Location: DEN.Airport, CO
Flight conditions: VMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E D01
ATC / advisory: TRACON D01
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: We were given a clearance by DEN ARTCC to descend and maintain FL270 approximately 40 NM prior to top of descent. I selected descend now on the descent page of the FMC. We were in VNAV path below the descent profile on the CREDE3 Arrival into DEN. The autopilot caught the descent profile and increased the rate of descent to make the appropriate restrictions on the arrival. Prior to TLRID intersection (17;000-15;000 feet restriction) we were sent over to DEN TRACON.We were trying to complete our approach checklist through FL180; check-in with Approach who immediately cleared us for the RNAV 16R Z Approach after CLFFF intersection. I brought CLFFF Intersection from the approach up just below CLFFF intersection on the arrival in the LEGS page of the FMC to close the discontinuity and verify the intersections for the approach when we got a 'caution terrain; caution terrain' aural warning from the GPWS and at about the same time DEN TRACON called us with a 'Low Altitude Alert.'My First Officer said 'we need to climb.' I immediately disconnected the autopilot; pushed the thrust levers up to emergency thrust and initiated a climb from just below 14;000 feet MSL to 15;000 feet MSL for the TLRID intersection crossing. DEN TRACON told us just to maintain 15;000 feet and we could either be vectored for a visual approach or go direct CLFFF for the RNAV 16R Z Approach and asked if we needed any help. We replied that we were having some difficulty with the VNAV up; but we were getting it sorted out and could go to CLFFF.Apparently the pitch mode had changed in the descent from VNAV to CWP (Control Wheel Pitch) somehow. I don't know if it did it when I closed the discontinuity with CLFFF intersection; or somewhere else. We flew the RNAV 16R and landed safely at DEN without further incident.Combine the Descent and Approach Checklists. Maybe add an item to the Approach Checklist to verify VNAV or mode in the Approach Checklist?


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737-700 flight crew reported receiving a GPWS terrain warning on arrival into DEN when they lost situational and mode awareness and were descending in control wheel pitch mode.